The present invention relates to a wireless communication device as defined in the preamble of claim 1. Such a wireless communication system can be a cellular phone, a cordless phone, a laptop computer with a wireless communication part, or the like.
A communication device of the above kind is known from the PCT patent application WO 91/02421. In a WO 91/02421, a receiver is described for digitally modulated signals in mobile communication systems. After quadrature mixing down of received radio signals to a lower frequency band, the mixed down quadrature signals are sampled and, after analog-to-digital conversion, stored for processing. The modulation of the signals can be Gaussian Minimum Shift Keying such as used in a cellular GSM (Global System for Mobile Communications) system, or any other suitable type of modulation. Before the received signal is mixed down, a coarse filtering of the radio frequency signal at the antenna is done by a tuneable filter/amplifier with an adjustable amplification factor. The known communication device is a so-called direct conversion receiver or zero-IF (Intermediate Frequency) in which the lower frequency band signals after mixing down are baseband signals. In WO 91/02421 the baseband signals after mixing down comprise a desired signal, signals from neighbouring channels and superimposed noise. A pair of lowpass filters for respective filtering of the mixed down quadrature signals is provided for filtering out the desired signal. After low pass filtering the quadrature signals are sampled, digitised and processed. From a receiver design point of view, the described receiver structure is preferred over conventional superheterodyne receivers because a direct conversion receiver in principle can be fully integrated.
With the design of a direct conversion receiver, special attention should be given to phase accuracy of the quadrature signals, to leakage problems with the local oscillator, and to problems caused by offset. Such a wireless communication device is not suitable as a multi-band receiving device because it is designed for receiving a particular frequency band only. To this end, all frequency selectivity measures are directed to obtain a proper selectivity for this particular frequency band only.
At present, in various parts of the world, different types of mobile radio systems are installed or are being installed, such as GSM systems, CDMA (Code Division Multiple Access) systems, and, in the US, D-AMPS systems (Digital Advanced Mobile Phone System). Such systems are operating in different frequency bands and desired signals are transmitted in channels of different bandwidths. In GSM, a full-duplex TD/FDMA (Time Division/Frequency Division Multiple Access) system, the mobile equipment""receive band is 925-960 MHz, the transmit band is 880-915 MHz and the channel spacing is 200 kHz. For CDMA (IS-95 Standard) these figures are 869-894 MHz, 824-849 MHz and 1250 kHz, and for D-AMPS the same bands as for CDMA with a channel spacing of 30 kHz. It can be seen from this figures that the frequency bands of the different systems are different, and, more importantly, that the bandwidth of the desired signals greatly varies. On top of variable bandwidth requirement, optimal filtering for the receiver depends on actual signal conditions. A more stringent band limitation to a desired signal minimum band width is desirable even at the cost of distorting the signal phase and information if the power level at neighbouring channels is high.
Because people are more and more travelling around the world and do want to use their mobile phone everywhere, there is a strong need for multi-band receiving devices. At present some multi-band receiving devices are onto the market. Most of these devices have multi-branch receivers, i.e., the receiver has parallel receiver branches which are optimised for each particular system. Sharing of hardware and software in such multi-band receivers is mostly after frequency conversion and sampling of the radio signals. Such devices are inefficient because there is a low level of hardware and software integration as regards radio functionality for different frequency bands.
It is an object of the present invention to provide a an efficient and cheap multi-band wireless communication device with a high performance.
To this end the wireless communication device according to the present invention is characterised by the features as defined in the claims. The present invention is based upon the insight that the bandwidth of the signal to be processed is an important parameter for obtaining an optimal receiver. If the bandwidth is wide enough, e.g. in the order of two or three times the information bandwidth of the desired signal, it is realised that the desired signal than passes the selection means with practically zero inter-symbol-interference. It was further realised that alternatively and equivalently the sampling time could be adapted to achieve the same optimal results. The principles of the invention can be applied to both a single band and a multi-band wireless communication device.
In one embodiment, bandwidth estimation is performed. Herewith, fully automatic adaptation in the receiver part itself is obtained. A priori knowledge about the currently received signals and system could also be acquired. Such a priori knowledge could be obtained via a system broadcast channel when the receiver is still in a non-optimal receiving mode. On the basis of such a priori knowledge, the selection means are controlled so as to put the receiver in an optimal reception mode for the current system.
In other embodiments, sampling means are used which are band limiting. Herewith, it is achieved that the sampling means do not add a substantial contribution to the pre-selected lower frequency band signals. This is achieved by effectively reducing the noise bandwidth of the pre-selected signal with a substantial factor. When switching ten capacitors in parallel, for instance, the noise bandwidth is reduced by a factor of ten. The same noise reduction can be achieved by a kind of averaging using digital processing means. The sampling rates at the input and output of the sampling means can be or are variable.
In a further embodiment, adaptive filtering is performed after sampling. Herewith, the receiver can dynamically adapt itself to varying adjacent channel to interference levels. Such a filter can be a Gaussion filter with an adaptive roll-off factor and/or bandwidth.
The receiver preferably is a direct conversion receiver. Herewith, the complete receiver could be integrated, possibly as a single chip receiver.